Historically, basestations used for wireless communications were designed to support a single air interface standard, for example AMPS (advanced mobile phone service), as the limited selection of air interface standards available in the cellular communications industry did not necessitate the use of more versatile basestations. More recently however, the emergence of several new air interface standards such as CDMA, TDMA and GSM made it desirable for a basestation to provide support to multiple air interface standards. To do this, the conventional approach consisted of providing designated signal processing equipment for each air interface standard to be serviced. The variety of signal processing functionality requirements of current cellular installations such as those used for PCS (personal communication system) operations which address a variety of air interface protocols do not permit a cost-effective and efficient allocation of the resources present at the basestation. Concurrent support of several air interfaces requires the duplication of wideband digital receivers and transmitters, A-D (analog-to-digital) and D-A (digital-to-analog) converters and signal processing equipment for each air standard serviced. The accommodation of several air interface standards necessitates replicating some of the apparatus used in both the receive and transmit directions for each additional air interface protocol sought to be supported by the basestation. The equipment unique to each protocol typically includes two sets of DSP (digital signal processing) units, an RX (receive) channelizer bank, a TX (transmit) channelizer bank, an RF front end which has a wideband receiver, a wideband transmitter and a pair of A-D (analog-to-digital) and D-A (digital-to-analog) converters as is well known in the art. However, duplicating this equipment for each air interface standard and more particularly the RF front end may very rapidly prove to have a major impact on the overall cost of the transceiver.
In addition to the need to reduce the cost of the transceiver, it would also be desirable to make the RF front end simple and more portable. This is due to the fact that some wireless service providers require the RF equipment to be remotely mounted on the antenna tower used for transmission with the remainder of the basestation installed on the ground so as to minimize losses between the antenna and the RF equipment.
In view of the possible remoteness of the RF front end section from the remainder of the basestation, the RF front end should be designed in a portable fashion to eliminate the need to install additional or different expensive and bulky RF equipment when different types of wireless signalling must be accommodated.
The need to address the multiple signalling protocols defined by the various air interface standards has also triggered the emergence of what is now known in the wireless industry as the software radio. A software radio is a highly desirable GCS (generic cell site) architecture for use in PCS installations. It consists of a single configurable basestation hardware design with signal processing equipment that can address several air interface standards. The particular features of interest of a software radio include software configurable wideband reception and transmission capabilities. These characteristics enable the software radio to handle multiple channels from various air interface standards. However, this software configurability does not allow multiple standards to be serviced simultaneously, instead allowing them to be serviced only in sequence.
U.S. Pat. No. 5,537,435 which issued Jul. 16, 1996 to Carney et al. and entitled "Transceiver Apparatus Employing Wideband FFT Channelizer With Output Sample Timing Adjustment And Inverse FFT Combiner For Multichannel Communication Network" discloses a multichannel wireless communication transceiver architecture for wideband signal processing. This patent discloses, among other things, the use of rational rate conversion techniques in an FFT-based wideband channelizer to provide optimum sampling of each digital channel signal output fed to the processing units. The transceiver architecture described in this patent uses a pair of oscillators from which a unique sampling rate for the A-D converter is selected according to the sampling requirements of the particular air interface standard serviced by the transceiver. In particular, the oscillators are provided to respectively accommodate TDMA or CDMA signal processing but the disclosure makes it clear that these two air interface protocols cannot be serviced simultaneously. The oscillators are merely provided to make the basestation more versatile in that it can selectively be configured to either process TDMA or CDMA signals.
U.S. Pat. No. 5,592,480 which issued Jan. 7, 1997 also to Carney and al. and entitled "Wideband Wireless Basestation Making Use of Time Division Multiple-Access Bus Having Selectable Number of Time Slots and Frame Synchronization to Support Different Modulation Standards" partially addresses the redundancy problem outlined above by the use of a TDM (time division multiplexing) bus for providing digital samples of a number of wireless communication channels. This TDM bus is claimed to efficiently service TDMA and CDMA standards simultaneously with a dynamic allocation of signal processing resources therefore eliminating the need to designate or install additional processing units for each air interface standard serviced. However, there is no solution provided for the increase in complexity in the RF front end (see FIG. 9 for example). Separate wideband digital tuners and exciters are required for each different air interface standard supported by the basestation.
Accordingly, there is a need for a cost-effective, simpler and more portable RF front end that can simultaneously service multiple air interface standards.